Holbæk Municipality is a kommune in northwestern Region Sjælland on the island of Zealand in Denmark. The municipality includes the island of Orø, and covers an area of 583 km², and has a total population of 73,440 (2023). Its mayor is Christina Krzyrosiak Hansen, a member of the Social Democrats and the youngest mayor ever in Denmark. The main town and the site of its municipal council is the city of Holbæk. On 1 January 2007 Holbæk municipality was, as the result of Kommunalreformen , merged with existing Jernløse, Svinninge, Tornved, and Tølløse municipalities to form a new Holbæk municipality.

Zealand at 7,031 km2 is the largest and most populous island in Denmark proper. Zealand had a population of 2,319,705 on 1 January 2020, comprising 40% of the country's population.